Optical geometry is a critical parameter in carrying out photometric analysis of various substances. For example, where a population of optical catheters are interchangeably used for transmitting light to and receiving reflected light from blood during measurements of oxyhemoglobin saturation (SO.sub.2) levels, the establishment of uniform geometry between the optical apertures of the transmitting and receiving fibers in each member of the optical catheter population permits a universal calibration to be performed for the entire population of optical catheters. A means for achieving such uniform optical geometry from catheter to catheter is disclosed in co-pending application Ser. No. 964,612, filed Nov. 29, 1978 now U.S. Pat. No. 4,295,470, issued Oct. 20, 1981 and assigned to the assignee of the present invention. Once an initial calibration is performed with one of the members of the optical catheter population constructed in accordance with Ser. No. 964,612, any of the members of the catheter population can be utilized to measure oxyhemoglobin saturation by simply standardizing the light transmissive properties of the selected optical catheter. The tip of the selected optical catheter is thereafter inserted into the blood flow of a patient and the remaining end of the catheter is connected to an oximeter of the type disclosed in U.S. Pat. No. 3,638,640 issued Feb. 1, 1972 to Shaw; U.S. Pat. No. 3,847,483 issued Nov. 12, 1974 to Shaw et al, or U.S. Pat. No. 4,114,604 issued Sept. 19, 1978 to Shaw et al.
Where the population of optical catheters are designed for in vivo insertion, suitable surgical procedures are employed to place one selected member of the optical catheter population in the blood stream of a patient. Thereafter, any remaining member of the population of optical catheters can be used interchangeably with the first selected member as previously indicated. Where extracorporeal determinations of oxyhemoglobin saturation levels are required, such as may occur during cardiopulmonary bypass (CPB) operations, conventional optical catheter techniques are less efficient. Extracorporeal in vitro measurements of SO.sub.2 can, of course, be obtained by inserting the tip of a selected optical catheter through appropriate tubing adaptors into the blood flowing through the cardiopulmonary bypass system. As is the case when in vivo measurements are involved, sterility, non-toxicity and cleaning considerations demand that catheters employed for in vitro measurements be disposed of after a single use. In contrast to the relatively long-term placement of optical catheters during in vivo SO.sub.2 level monitoring, however, this short-time once only use of optical catheters to obtain extracorporeal, in vitro SO.sub.2 measurements can prove unjustifiably expensive. Accordingly, a reliable and economically practical means for providing an external optical connection between photometric analyzing equipment and an extracorporeal sample of fluid would be of obvious advantage.